Feline Hyperthyroidism is one of the most common endocrine disorders in middle-aged and older cats. It is estimated that one in three hundred cats are affected by hyperthyroidism. The normal thyroid glands are located in the neck and produce hormones that regulate cellular metabolism in all organs of the body. The amount of thyroid hormone produced by the thyroid glands is regulated by the pituitary gland in the brain. The pituitary signals the thyroid glands to maintain proper levels of circulating thyroid hormone. This mechanism keeps cellular metabolism in balance. Most cases of feline hyperthyroidism are due to benign thyroid tumors called nodular hyperplastic goiters. These goiters are clones of autonomous cells that produce hormones but are not affected by pituitary regulation. Consequently, these tumors produce excessive amounts of thyroid hormone. Only the size and capacity of the tumor determine the amount of excessive hormone produced.

The signs and symptoms of hyperthyroidism are multi-systemic and relate to increased metabolic changes. Hyperactivity and other behavioral changes, weight loss, increased appetite, increased thirst, increased elimination, vomiting, diarrhea, panting, rapid heart rate, increased shedding, and occasionally lethargy have been observed. An enlarged thyroid mass(es) may be palpated in the lower neck area, although many cats have masses in the chest and no evidence of any masses in the neck. High blood pressure or hypertension, which affects highly vascular organs of the body, is a common symptom. Blindness due to retinal hemorrhage, kidney disease, heart disease (including heart failure), and hypertrophic cardiomyopathy may also be associated with this condition. Without treatment, these signs and symptoms will become more severe with time and produce fatal results.